The following is from Learn clojure in Y minutes.
The hashtag with brackets indicates a persistent hash set.
> (class #{1 2 3})
clojure.lang.PersistentHashSet
set - gets the set of values in a collection
> (set [1 2 3 1 2 3 3 2 2 1 3])
#{1 2 3}
conj - adds a member to a set
> (conj #{1 2 3} 4)
#{1 2 3 4}
disj - removes a member from a set
> (disj #{1 2 3} 1)
{2 3}
To test for existence of an item in a set, use the set as a function.
> (#{1 2 3} 1)
1
> (#{1 2 3} 4)
nil
There are plenty more functions in the clojure.sets namespace.